The 12-year-old boy had finished his homework and was playing a video game when he heard his mother cry out. Rushing to her aid, he found her on the kitchen floor, straddled by a fellow resident of their Prince George's County boarding house, the man's hands wrapped tightly around her neck, the boy said yesterday.

The boy said he grabbed a knife and swung, slashing 64-year-old Salomon Noubissie across the neck and opening an artery. Noubissie was fatally wounded.

The mother, Cheryl Stamp, said she did not immediately understand what had happened. "What did you do?" she said she asked her son.

"He didn't say anything," she said. "But I knew when I looked in his eyes. I said, 'Oh, Lord.' "

Law enforcement officials were reviewing evidence yesterday and had not decided whether to file charges. Their preliminary account of the incident broadly matches that of the boy and his mother.

The case presents exceedingly unusual circumstances: Rarely is a 12-year-old implicated in a homicide, and even less often does a child that age take a life to protect his mother.

"In Maryland, there can be a legitimate defense of third parties in the event of a violent attack," State's Attorney Glenn F. Ivey said. "That is a possibility in this case."

Yesterday, Stamp and her son were secluded in the boarding house on Roosevelt Avenue in the Landover area, curtains closed and doors locked against reporters and neighbors.

Like other neighbors, Turan Queen said she stood by the child. "His reaction was to help his mother," she said. "This was a 12-year-old defending his mother."

Stamp and her son agreed to be interviewed by Washington Post reporters, in part to explain the boy's actions. The Post is not naming the boy because he is a minor.

Efforts to contact Noubissie's family were unsuccessful.

Stamp said she and Noubissie, a Cameroonian immigrant, moved into the boarding house within days of each other about three months ago. They became fast friends, she said.

Stamp said that she is unemployed and that Noubissie had told her he was studying to be a psychiatrist. She said the boarding house is owned by Noubissie's nephew, a Massachusetts resident.

On Monday night, she said, Noubissie was not himself. He started to yell at her and grab her hair, she said. He was speaking in his native French, as he often did, but this time in "a devilish voice," she said. "He was talking crazy," Stamp said.

She said she tried to use "reverse psychology," ordering him to leave the kitchen and go to his room to calm down. His response was violent, she said.

"He threw me into the door so hard it hit my back, and it made my chest start hurting," she said. "Then he threw me to the floor. He threw me down and started choking me. I think that's when my son came in. . . . He protected me."

The boy, who is 5 feet 6 inches tall and weighs 175 pounds, said he acted because he thought he had no choice. "He was hitting her with the broom; then he was choking her," the boy said. "I told him to stop."

He grabbed a knife that he said his family had last used to cut turkey at Thanksgiving dinner.

"I knew I had to kill him so he would stop hurting my mother," he said.

Once she was freed, Stamp said she yelled upstairs for someone to call police. She said her son took her by the arm and led her into their bedroom.

Nearby, Noubissie was flailing and yelling, Stamp and her son said. As the door closed, she noticed the blood coming from his neck. "I didn't know where all that blood was coming from," she said. "He was talking in that language -- loud."

Stamp said she did not realize for several moments that her son, and not she, had been responsible for inflicting the injury that caused Noubissie to release her.

In the bedroom, as they waited for police, the boy did not speak, Stamp said. She sat on a couch, looked down and saw the bloody knife, she said.

Noubissie was alive when police arrived, Stamp said. He was combative with the officers, she said, even as he bled heavily. He died at a hospital. Police sources confirmed her account.

Stamp, who has two adult children and a 17-year-old daughter who lives with the girl's father, said the tragedy was the second to befall her family. She supports herself and her son from "widow's benefits" she has received since her husband fatally shot himself more than 20 years ago. Her eldest son, 27, witnessed the suicide, she said. "I've had enough drama in my life," she said.

The 12-year-old boy said yesterday that he was not happy about what he had done but that he knew that it was the right thing.

"I just asked God again to protect me and my mother," he said. "I told God that I had stabbed him because he was killing my mother. I know he understands, and I think he will keep us safe now."

After the stabbing Monday night, after police had left and neighbors returned to their homes, the two sat and held each other. There was no sleep that night for either.